In the design of high-speed I/O circuits such as USB interfaces or SATA interfaces, it is necessary to use a precision voltage and a precision current as references for impedance matching. Please refer to FIG. 1, which is a diagram illustrating a reference circuit capable of providing a precision voltage and a precision current according to prior art. As shown, an IC circuit 10 includes a bandgap voltage reference circuit 12, an operational amplifier 14, a mirroring circuit 16, a transistor M1, and an I/O pad 18.
Generally speaking, the bandgap reference circuit 12 is used for providing a stable bandgap voltage (VBG), which will not change as the manufacturing process, the temperature or the supply voltage changes. Therefore, the bandgap voltage VBG outputted by the bandgap voltage reference circuit 12 can be viewed as a precision voltage. As shown in FIG. 1, the bandgap voltage VBG is inputted to a positive input terminal of the operational amplifier 14, and a negative input terminal of the operational amplifier 14 is connected to the I/O pad 18 of the IC circuit 10. In addition, the drain of the transistor M1 is connected to a first terminal of the mirroring circuit 16, the gate of the transistor M1 is connected to the output terminal of the operational amplifier 14, and the source of the transistor M1 is connected to the I/O pad 18 of the IC circuit 10. The IC circuit 10 further utilizes an external precision resistor RP connected between the I/O pad 18 and ground.
Obviously, when the operational amplifier 14 operates normally, the voltage at the I/O pad 18 of the IC circuit 10 will be the bandgap voltage VBG and thus a first current I1 flowing through the external precision resistor RP is (VBG/RP). In addition, this first current I1 is outputted through the first terminal of the mirroring circuit 16, and the second terminal of the mirroring circuit 16 can also output a reference current Iref, which is directly proportional to the first current I1 and can be viewed as a precision current. In other words, the intensity of the precision current can be determined according to the resistance of the external precision resistor RP.
According to the prior art, in order to obtain both the precision voltage and the precision current in the same circuitry, the I/O pad 18 is designed in the IC circuit 10 and connected to the external precision resistor RP to generate the precision current. In other words, an external precision resistor is required and needs to be additionally disposed on the circuit board, which results in inefficient problems in space and cost.
In addition, due to the I/O pad 18 being designed in the IC circuit 10, the designer of the IC circuit 10 must design an electrostatic discharge protection circuit (ESD) to protect the I/O pad 18. Accordingly, the layout area of the IC circuit 10 is increased. If the I/O pad 18 is disposed in the IC circuit 10, another problem of generating noise on the I/O pad 18 might be caused.
Furthermore, the stability of the operational amplifier 14 is decided by its phase margin. If the operational amplifier 14 is unstable, the parasitic capacitance on the I/O pad 18 is hard to be estimated, which might result in loop instability and loop oscillation.
In order to obtain the precision voltage and the precision current, a reference voltage distribution system is disclosed in the International Patent Application No. PCT/US90/05473. This system generates a precision current according to an external reference voltage and a controllable resistance. However, this system needs an additional control circuit for controlling the resistance.
In addition, a dual source for constant current and PTAT (proportional to absolute temperature) current is disclosed in the International Patent Application No. PCT/US96/18048, wherein a bandgap voltage reference circuit is used to generate a bandgap reference voltage (VBG) and a PTAT voltage (VPTAT), and thereby generate the precision current and the PTAT current. Likewise, an external precision resistor is still needed in order to generate the precision current and the PTAT current.
Moreover, in the periodical “IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS”, vol. 50, no. 12, Dec. 2003, a new low voltage precision CMOS current reference circuit with no external components is proposed. Please refer to FIG. 2. FIG. 2 is a diagram illustrating a circuitry disposed in an IC circuit and capable of providing a precision current according to the prior art. The IC circuit 30 includes a bandgap voltage reference circuit 32 with a positive temperature coefficient, an operational amplifier 34, a mirroring circuit 36, and transistors M1, M2 and M3.
The bandgap voltage reference circuit 32 with positive temperature coefficient is used for providing a temperature-dependent bandgap voltage (VBG), which increases as the temperature rises. As shown in FIG. 2, the bandgap voltage VBG is inputted to the positive input terminal of the operational amplifier 34, and the negative input terminal of the operational amplifier 34 is connected to the drain of the transistor M1. In addition, the drain of the transistor M3 is connected to a first terminal of the mirroring circuit 36, the gate of the transistor M3 is connected to the output terminal of the operational amplifier 34, and the source of the transistor M3 is connected to the drain of the transistor M1. The source of the transistor M1 is grounded, and the gate of the transistor M1 is connected to the gate of the transistor M2. The source of the transistor M2 is grounded, and the gate and the drain of the transistor M2 are connected to a second terminal of the mirroring circuit 36.
In the IC circuit 30, the transistor M1 has to be operated in a triode region and the transistor M2 has to be operated in a saturation region to make the transistor M1 exhibit a feature of negative temperature coefficient. Hence, by collocating the bandgap voltage (VBG) with the positive temperature coefficient and the transistor M1 with the negative temperature coefficient, a precise first current I1 can be generated. In addition, with the first current I1 being outputted from the first terminal of the mirroring circuit 36, a reference current Iref is outputted from the second terminal of the mirroring circuit 36 The reference current Iref is directly proportional to the first current I1 and can be viewed as a precision current.
Although providing a precision current, the abovementioned circuitry does not provide any precision voltage. Hence, an additional bandgap voltage reference circuit is required to provide a temperature-independent bandgap voltage (VBG). In addition, due to possible deviations rendered by mass production in the manufacturing process of the IC circuit, it is difficult to control the transistor M1 to be operated in the triode region.